I have attended both weekday-only schools and classes on weekends.
Some of my thoughts:
Do the instructors have time in their schedules?
Do the students' demographics make the time available?
What is the "culture" and tradition?
What is the weather?
Weather you say - in some parts of Northern USA you may at best have only ten-to-twelve great summer weekends potential - exluding the rainouts.
So it tough call - many times Saturday classes are well attended for nine months a year and then -
Whamo - summer comes in. I know of places that shut down their Saturday classes (esp. children centric) when the academic calendar ends and pick it back up in September.
Many more adult oriented MA classes - such as JSA and Aikido - often have a strong tradition of weekend classes.
My dream - and I tried it - was to offer Sunday evening classes. What the hell are you going to do on a typical Sunday evening say at Seven P.M. that is worthwhile? Well unless it is FOOTBALL season.
BTW it seems most adults are too tired by the time Sunday evening arrives. Wussies!